1. Field of the Invention
This invention relates to improvements in variable speed motor controllers for a 3.0. AC motor and more particularly to improvements for reducing the amount of energy expended to produce the desired speed and output torque of the motor at less than full load conditions over at least a portion of the speed range controlled by the controller.
2. Description of the Prior Art
Variable speed motor controllers of the type shown herein for 3.0. AC motors without the improvement of this invention have been produced for many years. Some of these variable speed motor controllers are sold by the assignee of this invention, and include a converter circuitry for deriving three phase AC voltages from a DC power source to drive the 3.0. AC motor windings. The frequency of the AC voltage is varied by a speed control to set the speed of the motor, and the magnitude of the AC voltage is also adjusted by the speed control to provide the current required by the motor to produce the desired torque. In these systems the AC voltage applied to the motor is increased by the speed control substantially linearly with speed. Thus under light loads, excess voltage is being provided to the motor which results in energy being wasted.
While this was at one time not a significant problem, the higher costs of electrical energy have necessitated that energy be saved whenever possible.
Since a large number of 3.0. AC motors are often operated at less than full load, it is desired to provide an inexpensive system to effect a reduction in excess motor voltage when the motor is operating at less than full load.